Viking reports a positive result. Something is ingesting the nutrients, metabolising them, and then belching out gas laced with carbon-14.

So why no party?

Because another instrument, designed to identify organic molecules considered essential signs of life, found nothing. Almost all the mission scientists erred on the side of caution and declared Viking's discovery a false positive. But was it?

The arguments continue to rage, but results from NASA's latest rovers show that the surface of Mars was almost certainly wet in the past and therefore hospitable to life. And there is plenty more evidence where that came from, Levin says. "Every mission to Mars has produced evidence supporting my conclusion. None has contradicted it."

Levin stands by his claim, and he is no longer alone. Joe Miller, a cell biologist at the University of Southern California in Los Angeles, has re-analysed the data and he thinks that the emissions show evidence of a circadian cycle. That is highly suggestive of life.

Levin is petitioning ESA and NASA to fly a modified version of his mission to look for "chiral" molecules. These come in left or right-handed versions: they are mirror images of each other. While biological processes tend to produce molecules that favour one chirality over the other, non-living processes create left and right-handed versions in equal numbers. If a future mission to Mars were to find that Martian "metabolism" also prefers one chiral form of a molecule to the other, that would be the best indication yet of life on Mars.

7 Tetraneutrons

FOUR years ago, a particle accelerator in France detected six particles that should not exist (see Ghost in the atom). They are called tetraneutrons: four neutrons that are bound together in a way that defies the laws of physics.

Francisco Miguel Marquès and colleagues at the Ganil accelerator in Caen are now gearing up to do it again. If they succeed, these clusters may oblige us to rethink the forces that hold atomic nuclei together.

The team fired beryllium nuclei at a small carbon target and analysed the debris that shot into surrounding particle detectors. They expected to see evidence for four separate neutrons hitting their detectors. Instead the Ganil team found just one flash of light in one detector. And the energy of this flash suggested that four neutrons were arriving together at the detector. Of course, their finding could have been an accident: four neutrons might just have arrived in the same place at the same time by coincidence. But that's ridiculously improbable.

Not as improbable as tetraneutrons, some might say, because in the standard model of particle physics tetraneutrons simply can't exist. According to the Pauli exclusion principle, not even two protons or neutrons in the same system can have identical quantum properties. In fact, the strong nuclear force that would hold them together is tuned in such a way that it can't even hold two lone neutrons together, let alone four. Marquès and his team were so bemused by their result that they buried the data in a research paper that was ostensibly about the possibility of finding tetraneutrons in the future (Physical Review C, vol 65, p 44006).

And there are still more compelling reasons to doubt the existence of tetraneutrons. If you tweak the laws of physics to allow four neutrons to bind together, all kinds of chaos ensues (Journal of Physics G, vol 29, L9). It would mean that the mix of elements formed after the big bang was inconsistent with what we now observe and, even worse, the elements formed would have quickly become far too heavy for the cosmos to cope. "Maybe the universe would have collapsed before it had any chance to expand," says Natalia Timofeyuk, a theorist at the University of Surrey in Guildford, UK.

There are, however, a couple of holes in this reasoning. Established theory does allow the tetraneutron to exist - though only as a ridiculously short-lived particle. "This could be a reason for four neutrons hitting the Ganil detectors simultaneously," Timofeyuk says. And there is other evidence that supports the idea of matter composed of multiple neutrons: neutron stars. These bodies, which contain an enormous number of bound neutrons, suggest that as yet unexplained forces come into play when neutrons gather en masse.

8 The Pioneer anomaly

THIS is a tale of two spacecraft. Pioneer 10 was launched in 1972; Pioneer 11 a year later. By now both craft should be drifting off into deep space with no one watching. However, their trajectories have proved far too fascinating to ignore.

That's because something has been pulling - or pushing - on them, causing them to speed up. The resulting acceleration is tiny, less than a nanometre per second per second. That's equivalent to just one ten-billionth of the gravity at Earth's surface, but it is enough to have shifted Pioneer 10 some 400,000 kilometres off track. NASA lost touch with Pioneer 11 in 1995, but up to that point it was experiencing exactly the same deviation as its sister probe. So what is causing it?

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The Hubble Deep Field. These distant galaxies are racing away from us far faster than theory predicts (Image: NASA)